Continue working on paragraph base direction.
Support per-buffer default paragraph direction.
buffer.h (struct buffer): New member paragraph_direction.
buffer.c (init_buffer_once): Initialize it.
(syms_of_buffer): Declare Lisp variables
default-paragraph-direction and paragraph-direction.
dispextern.h (struct it): New member paragraph_embedding.
xdisp.c (init_iterator): Initialize it from the buffer's value
of paragraph-direction.
<Qright_to_left, Qleft_to_right>: New variables.
(syms_of_xdisp): Initialize and staticpro them.
(set_iterator_to_next, next_element_from_buffer): Use the value of
paragraph_embedding to determine the paragraph direction.
bidi.c (bidi_line_init): Fix second argument to
bidi_set_sor_type.
(bidi_init_it): Initialize paragraph_dir to NEUTRAL_DIR.
(bidi_get_next_char_visually): Record the last character of the
separator in separator_limit, not the character after that.
(bidi_find_paragraph_start): Accept character and byte positions
instead of the whole iterator stricture. All callers changed.